As will be appreciated herein below, except as otherwise indicated, aluminium alloy designations refer to the Aluminium Association designations in Aluminium Standards and Data and the Registration Records, as published by the Aluminium Association in 2013 and are well known to the person skilled in the art.
For any description of aluminium alloy compositions or preferred aluminium alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
Aluminium alloys comprising lithium are very beneficial for use in the aerospace industry since the purposive addition of lithium may reduce the density of the aluminium alloy by about 3% and increase the modulus of elasticity by about 6% for each weight percent of lithium added. In order for these alloys to be selected in airplanes, their performance with respect to other engineering properties must be as good as that of commonly used alloys, in particular in terms of the compromise between the static mechanical strength properties and the damage tolerance properties. Over time a wide range of aluminium-lithium alloys have been developed with a corresponding wide range of thermo-mechanical processing routes. However, a key processing route remains the casting of ingots or billets for further processing by means of extrusion, forging and/or rolling. The casting process has proven to remain a problematic processing step in the industrial scale production of ingots and billets. There are, for example, issues with regard to oxidation of molten metal in the furnaces, the transfer troughs and during casting itself.
U.S. Pat. No. 4,761,266 (assigned to Kaiser Aluminum) discloses a method for preparing an aluminium-lithium alloy at a preselected ratio of aluminium to lithium. The method comprises preparing an amount of molten lithium and an amount of molten aluminium melt. The molten lithium is filtered using stainless steel filters to remove solids from the molten lithium, notably lithium oxides and hydroxides. The molten aluminium melt is melt treated by degassing prior to mixing with the molten lithium. The molten lithium and molten aluminium are mixed in a complex apparatus incorporating a vortex bowl. The swirling action of the vortex causes mixing of the aluminium and lithium, which then proceeds as a homogeneous mixture downward through an exit passage at the base of a funnel. The mixture enters a degassing chamber, where the mixture is purged with argon. The purged mixture is then passed through a filter to remove any oxides and refractory fragments which may have entered the system. The molten mixture then enters an ingot casting station. All components of the system are blanketed in an inert atmosphere. This method has various disadvantages. For example, there is a sensitivity for viscosity of the alloy and thus for fluctuations in the temperature of the metal in the vortex bowl. Although the system is blanketed in an inert atmosphere, there will be a high risk of entrapment of gas and oxides in the molten metal, which have to be removed subsequently. The alloying system is a complex and dynamic system whereby small variations in metal flow may lead to undesirable changes in alloy composition in the final ingot.